International Standard IEC 61800 is relating to properties of adjustable speed electrical power drive systems, thus including rail mounted gantry cranes (RMG), automatic stacking cranes (ASC), ship-to-shore cranes (STS), quay cranes and trolleys used in said cranes. Particularly in the part 5-2 of said Standard are defined the functional safety requirements for said drive systems. In the International Standard IEC 61800-5-2 the stop and brake functions of the adjustable speed electrical power drive systems are defined and divided in categories as follows:                Stop category 0: An uncontrolled stop in which the stopping is performed by immediate removal of power to the hoisting machine actuators.        Stop category 1: A controlled stop with power available to the hoisting machine actuators. When the stop is achieved power is removed from the actuators.        Stop category 2: A controlled stop with power left available to the hoisting machine actuators.        
In stop categories 1 and 2 the braking and stop of the crane and/or trolley will happen in a controlled manner without any major problems. Problems will occur especially in stop category 0 braking which in fact is an emergency braking. This kind of situation happens e.g. when a person is detected in the operating range of an automatic stacking crane (ASC), whereby all operations of the crane are stopped immediately. In this type of braking a mechanical brake is switched on, said brake being always on with a constant torque. Normally the mechanical brake would be used only as a parking brake. Even though this type of braking is not happening according to a normal operating mode, in some circumstances it may occur frequently, e.g. ten times a day.
So a crane or a trolley is a machine running on rails the wheels of which machine will be locked and sliding on the rails during stop category 0 braking. The sliding of the wheels causes wheel damaging. When the wheels are sliding on the rails extremely high temperatures are formed to small areas on wheel surface causing martensitic reaction which weakens the wheel strength. Due to this phenomenon the wheel surface may in worst cases come off, at least partially. Better materials for wheels as well as other mechanical solutions have been searched and examined but current solutions and materials have been found best available at least at the moment.
One prior art braking system is disclosed in U.S. Pat. No. 5,573,312. This document discloses a vehicle having an electric and mechanical braking system with which a braking wheel can be rotated by a motor. The mechanical brake of this system, however, is dynamic so that it does not provide a constant but variable torque. Also the vehicle itself is of a type different from the inventive one and it cannot be seen that this prior art system could be applicable in RMG cranes, especially in ASC cranes.